While PD-1 blockade is effective in relapsed or refractory classical Hodgkin lymphoma (cHL) and in a subset of diffuse large B cell lymphomas (DLBCL) patients, a majority of B cell lymphoma patients do not respond to such anti-PD-1 immunotherapy. Hence, it is necessary to develop new therapeutic strategies to enhance responses to mmunotherapy. However, it remains poorly understood how to enhance tumor immunogenicity to improve anti-tumor immune responses. In this regard, histone deacetylase (HDAC) inhibitors may serve as an attractive means to achieve such goals, e.g., by upregulating major histocompatibility complex (MHC) in cancer cell lines and/or modulating immune cells' functions. Nevertheless, most HDAC inhibitors (HDACi) tested so far are not very effective in treating cancers as a single agent; this may be due to the fact that different HDACi can cause highly variable biological effects in cancer or normal cells. Furthermore, although the cytotoxic effects of HDACi on tumor cells have been studied extensively, it remains poorly understood how HDACi affect immune cells and what precise role HDACi have in anti-tumor immunity. Importantly, it remains largely unknown how to stratify cancers to identify the HDACi sensitive subtypes. In this application, we propose to elucidate the mechanisms by which a newly developed HDACi sensitizes B cell lymphomas to PD-1 therapy, and identify biomarkers that predict the efficacy of combined treatment of HDACi and anti-PD1 antibody. Hence, our proposed studies may advance the field one step forward by developing novel therapeutic strategies to target cancers resistant to PD-1 blockade due to reduced immunogenicity. We recently established a unique mouse model of G1XP lymphomas by lineage-specific deletion of Xrcc4, a DNA repair gene, and Trp53 in activated B cells. Our preliminary data show that G1XP lymphomas resemble the features of human B cell lymphomas and provide a unique experimental platform for testing new therapies that target lymphomas with reduced immunogenicity. Our objective here is to develop novel combinatorial strategies to treat aggressive B cell lymphomas. To do so, we will test our hypothesis using mouse B cell lymphoma models, humanized mouse models for B cell lymphoma, and our newly developed HDACi.